Investigation of the fusion mechanism of human immunodeficiency virus (HIV) and its target cells is vital to understanding the acquired immunodeficiency syndrome (AIDS) and its prevention and treatment. The fusion process involves two envelope glycoproteins, gp120 and gp41, on HIV and two receptors, CD4 and a chemokine receptor, on the target cell. Previously we have reported that viral fusion peptides can penetrate cells susceptible to infection by their corresponding viruses and serve as carrier of antiviral agent or as probe for the components involved in the virion-cell fusion. By using various reagents such as chemokines, antibodies, peptide fragments from gp120 and gp41, or by expressing receptors with mutated domains, the fusion peptide conjugated with a reporter peptide can detect the structural requirements for viral entry into cells. The resultant information is useful to the design of antiviral drugs or vaccines. The present study is an attempt to resolve several puzzling problems observed in the past. Although the HIV gp41 fusion peptide generally can gain entry into cells expressing chemokine receptors like CXCR4, CCR5, and CCR6, it fails to enter a few cell lines like Hos and U87 expressing these receptors. Initial results indicate that in some cases the impeded entry is due to low level of chemokine receptor expression. Another possibility, as has been shown in the literature, is that the chemokine receptor is physically linked to another receptor such as CD4, thereby requiring an additional factor like gp120 for entry. Collaboration with Dr. Zhennan Lai has been initiated in order to take advantage of the gene transfer technigue utilizing lentiviral vectors, developed in Dr. Rosco O. Brady's laboratory, to increase the level of receptor expression. Entry of the fusion peptide into HeLa and NIH3T3 cells sometimes also fails, causing concerns about the cellular entry model. In addition, the fusion peptides per se seem to cause abnormal cell growth which leads to lack of cellular entry. These problems in some cases can be overcome by adjusting the concentration of fusion peptides used or the length of its incubation time with the cells. It appears, however, that the proposed involvement of chemokine receptors functioning as the entry point for gp41 remains correct.